Bump web-ext in /glean/tests/platform/utils/webext/sample

Bumps [web-ext](https://github.com/mozilla/web-ext) from 5.5.0 to 6.1.0.
- [Release notes](https://github.com/mozilla/web-ext/releases)
- [Commits](mozilla/web-ext@5.5.0...6.1.0)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]